1. Field of the Invention
The present invention relates to positive feed tools, such as right angle positive feed drills, and more particularly, to a method and apparatus for limiting the transmission of power along a drive train of such tools and for causing the spindle of such tools to automatically retract when torque on the spindle or in the drive train exceeds a predetermined value.
2. Description of the Related Art
Positive feed tools, such as positive feed drills, are conventionally known for drilling holes in workpieces formed of substances, such as steel, aluminum, titanium, and composites. Positive feed drills include a drill feed mechanism that feeds a drill bit into the workpiece.
FIG. 1 illustrates an example of a conventional positive feed drill, specifically a right angle positive feed drill 10 that is coupled to a cutter 12. The positive feed drill 10 generally includes a spindle 28 that, in addition to rotating, advances a predetermined amount per revolution toward the workpiece to be drilled. Conventional applications for positive feed drills include, among other applications, drilling holes in various parts of aircraft.
The right angle positive feed drill 10 includes an air motor 14. The air motor 14 is powered by a pressurized air source (not illustrated). As described below, the air motor 14 causes the spindle 28 to rotate. The spindle 28 is caused to rotate and feed by rotating the spindle drive gear 18 and spindle feed gear 20 with a differential feed gear 24 and differential drive gear 26. The spindle feed gear 20 includes internal threads that are threaded on the external threads 27 extending along the length of the spindle 28. Hence, when the spindle feed gear is rotated in relation to the spindle 28, the spindle will feed through the spindle feed gear. External threads 27 of the spindle 28 illustrated in FIG. 1 are left-handed threads. The spindle 28 also includes drive grooves 30 that extend along the length of the spindle 28. The spindle drive gear 18 includes internal male splines (not illustrated in FIG. 1) that engage with the drive grooves 30 on the spindle 28. Thus, when the spindle drive gear 18 is rotated, the spindle 28 also rotates.
When the air motor 14 is actuated, spindle drive gear 18 is caused to rotate, which will turn the spindle 28 due to the engagement of the internal male splines with the drive grooves 30. In forward operation, or the drilling mode, the air motor 14 turns in a clockwise direction (as viewed from the rear of the tool 10), which turns a motor spindle 16. The series of gears 32, 34, 38, 40, 26 connect the motor spindle 16 with the spindle 28. More specifically, rotation of the motor spindle 16 will rotate the pinion 32, which in turn drives the gear 34, which is pinned or keyed to a shaft 36. The spur pinion 38 drives the idler gear 40, which drives the differential drive gear 26. In forward drill mode, the differential drive gear 26 is coupled to the differential feed gear 24 so that they turn in unison. The differential drive gear 26 is also engaged with the spindle drive gear 18. Because the spindle drive gear 18 is engaged with the spindle 28 via the drive grooves 30, the rotation of the differential drive gear 26 is transferred to the spindle 28. However, the spindle 28 is permitted to move longitudinally through the spindle drive gear 18 because of the drive grooves 30.
The spindle feed gear 20, which is threaded on the spindle 28, is driven by the differential feed gear 24 while in the forward position, as shown in FIG. 1. The spindle feed gear 20 threads the spindle 28 through the spindle drive gear 18 and feeds it toward the workpiece. Because a differential exists between the spindle drive gear 18 and the spindle feed gear 20, the spindle 28 is rotated and will advance toward the workpiece. The desired feed rate is obtained by the differential gear ratio between the spindle drive gear 18 and the spindle feed gear 20. In sum, when the air motor 14 is actuated, the spindle drive gear 18 rotates, which turns the spindle 28. When the spindle feed gear 20 is rotated faster than the spindle 28, the spindle will feed, causing downward motion of the spindle. Conversely, when the spindle feed gear 20 rotates slower than the spindle 28, the spindle 28 will retract upward.
The right angle positive feed drill 10 also includes a feed stop collar 42 and a feed engagement lever 44. At the completion of the advancement of the spindle 28, or at the completion of the drilling cycle, the feed stop collar 42 contacts the feed engagement lever 44. This contact lifts the differential feed gear 24 away from the differential drive gear 26 and locks it so that it does not rotate. Because the differential feed gear 24 is locked and is engaged with the spindle feed gear 20, the spindle feed gear 20 is also locked in a stationary position such that it does not rotate. With the spindle 28 continuing to rotate in a forward direction via rotation of the spindle drive gear 18, and the spindle feed gear 20 held stationary, the spindle 28 will retract.
As illustrated in FIG. 1, the cutter 12 includes a drill bit 45 for penetrating the surface of the workpiece to be drilled. A tool nose 46 surrounds the cutter 12, which attaches the tool to a drilling fixture offset from the workpiece to be drilled. The drill bit 45 is a tool that bores cylindrical holes.
During operation of the conventional tool illustrated in FIG. 1, it is possible that the spindle 28 will seize during operation of the air motor 14. This could occur for a variety of reasons.
For example, during drilling with the drill bit 45, the metallic chips created during the cutting operation may stick to the wall of the bored cylindrical hole and gall. This will cause the cutting chips to stick to the cutting edge of the drill bit 45, as well as the wall of the drilled hole. When this occurs, the drill bit 45 may seize in the drilled hole, which naturally causes the spindle 28 to seize as well. This may occur because the drill bit 45 does not have enough flutes for the release of the cutting chips.
Additionally, when the drill bit 45 breaks through a workpiece, it occasionally grabs an edge of the hole and seizes. Furthermore, because the tool nose 46 surrounds the drill bit 45, chips from the drilling operation tend to pack in the tool nose 46 to such an extent that the drill bit 45 seizes.
It is particularly problematic when the drill bit 45 or the spindle 28 of the right angle positive feed drill 10 seizes. During operation of the drill 10, the air motor 14 is supplying power through the gear train to the spindle 28. When the spindle 28 seizes while the motor 14 is attempting to drive the spindle 28, a high torque situation inevitably results in the positive feed drill 10, which will cause the air motor 14 to seize, or damage one or more of the gears, bearings, and shafts in the drill 10. For example, if the spindle 28 seizes during normal drilling with the drill bit 45, the spindle 28 and/or any one of the pinions and gears may strip or completely break and damage the tool 10.
One conventional right angle positive feed drill 10 similar to that illustrated in FIG. 1 has the ability to rapidly advance the drill bit 45 from a retracted position to a position near the workpiece. This feature is not used to drill holes, but to merely cause the drill bit 45 to advance quickly toward the workpiece. This rapid advance feature is disclosed in both U.S. Pat. No. 4,799,833 and U.S. Pat. No. 4,591,299 and is achieved as follows.
With the spindle 28 retracted upwardly, the air motor 44 is reversed. With the differential feed gear 24 located in the upward position where it is locked so that it does not engage with the differential drive gear 26, the spindle feed gear 20 is also stopped and does not rotate. With the motor 14 running in reverse, the spindle drive gear 18 rotates the spindle through the internal threads of the spindle feed gear 20 in a reverse direction. This will cause the spindle 28 to rapidly advance toward the workpiece.
In another conventional right angle positive feed drill similar to that illustrated in FIG. 1, the drill includes a clutch that will slip or disengage when the drill encounters excessive torque during a rapid advance cycle with the motor running in reverse. The clutch is of a conventional type and is oriented perpendicular to an axis of the air motor. See U.S. Pat. No. 4,799,833. However, this clutch is configured to only trip when the air motor 14 is operating in reverse and the spindle is rapidly advancing toward the workpiece. Thus, when the air motor is rotating in the normal forward direction to feed the spindle 28 toward the workpiece during normal drilling, the clutch of the conventional right angle positive feed drill will not slip or disengage when the drill encounters excessive torque during the drilling cycle. Hence, with this conventional drill, the spindle, gears, shafts, bearings, or other portions of the drill may be damaged during the drilling operation should a high torque situation occur, such as when the drill bit seizes.
Even more problematic, some conventional positive feed drills do not include an automatic retract mechanism. With these tools, when the depth stop 42 bears down against the housing of the drill, the spindle teeth or the gear teeth may strip. Hence, an operator of such drills must continuously monitor the operation of the drill to ensure that the depth stop does not torque on the housing of the tool.
To address the above-described problems, some positive feed drills include a shear pin in the tool that will shear when the tool is subjected to excessive torque. This approach to solving the above problems is problematic because the shear pin does not always function reliably and must be replaced before the tool can be used again.
Thus, it is apparent that conventional positive feed drills are particularly vulnerable to being damaged when subjected to a high torque situation during drilling. These positive feed drills may be permanently damaged if the operator does not immediately cause the drill bit 44 to retract when a high torque situation occurs during drilling. Hence, an operator of conventional positive feed drills must continuously monitor the drilling operation to determine whether or not a high torque situation could possibly occur, and when this is observed, stop the drill and remedy the situation. However, it is very difficult for an operator to determine when a high torque situation is developing and thus when the drill should be shut down.
From the foregoing, it is apparent that the above-described constraints and problems associated with conventional positive feed tools has created a need for a positive feed tool having a clutch that will trip in response to a high torque situation during drilling, as well as a positive feed drill that automatically retracts the spindle away from the workpiece in response to the high torque situation.